In magnetic resonance signals measured in MRI, there is observed the chemical shift phenomenon, which means small differences of resonance frequency originated in difference in molecular structure. There are known MRS (Magnetic Resonance Spectroscopy), in which magnetic resonance signals are separated for every molecule (metabolite) by using the chemical shift phenomenon to obtain spectra, as well as CSI (Chemical Shift Imaging) and MRSI (Magnetic Resonance Spectroscopic Imaging), in which spatial signal intensity distribution is imaged for every metabolite.
Major metabolites of human bodies detectable by MRS or MRSI include choline (Cho), creatine (Cr), N-acetylaspartic acid (NAA), lactic acid (Lac), and so forth. Amounts of these metabolites enable stage determination and early diagnosis of metabolic disorders such as cancers. Moreover, it is also considered that they enable noninvasive diagnosis of malignancy of tumors.
In MRS and MRSI, eddy currents are induced by gradient magnetic fields applied at the time of measurement. Eddy currents cause spatially and temporally uneven static magnetic fields to distort shapes of spectra obtained by the measurement. This spectral distortion is usually corrected by using phase values of signals of a substance showing higher signal intensities compared with metabolites. For example, water is used as the substance showing higher signal intensities compared with metabolites (refer to, for example, Non-patent document 1). According to the method disclosed in Non-patent document 1, spatial and temporal phase values are calculated from FID (Free Induction Decay) signals of water, and phase correction is performed for metabolite image data to correct the spectral distortions induced by eddy currents.
In CSI or MRSI, size of measurement matrix (voxel number) is very small, as small as about 8×8 to 32×32, in view of measurement time and SNR (signal to noise ratio). Therefore, truncation is caused by the Fourier transform performed in the image reconstruction to cause contamination of signals of distant voxels. As a result, unevenness of static magnetic field causes contamination of water signals having a frequency different from that of the water signals in the objective voxels.
Contamination of water signals of such a different frequency generates so-called phase jump regions in time change of the phase values used for the eddy current correction. The phase jump regions are regions where variations of change amounts of phase values per unit time are outstandingly larger compared with the other regions. Magnitude of the phase value change in the phase jump regions is proportional to concentration of the contaminated water signals. Such phase jump will be explained below with reference to FIG. 17.
FIG. 17A shows water signal spectra obtained by computer simulation. The curves of Water 1 and Water 2 shown in FIG. 17A represent spectra of water signals having frequencies of 2 Hz and 5 Hz, respectively, and a concentration ratio of 1.0:0.9. Temporal change of phase value of the FID signal of water in the time domain in the presence of these two kinds of signals is shown in FIG. 17B. As shown in FIGS. 17A and 17B, when the frequency difference is Δf, there are generated phase jumps proportional to the concentration ratio with a time interval of 1/Δf. In this example, mild phase change is induced by adding static magnetic field change caused by an eddy current.
Spectra obtained before and after performing correction of spectral distortion caused by eddy currents of metabolites using phase values showing the change shown in FIG. 17B for the time direction are shown in FIGS. 17C and 17D, respectively. The spectrum observed before the eddy current correction is shown in FIG. 17C, and the spectrum obtained after the eddy current correction is shown in FIG. 17D. As shown in FIG. 17D, if the correction is performed by using phase values including phase jumps, ringing artifacts are generated by the phase jumps, and thus the spectrum is degraded by the eddy current correction to the contrary.
As a countermeasure for the above problem, there is, for example, a method of reducing ringing artifacts by passing the spectrum of water signal through a low pass filter (refer to, for example, Non-patent document 2). There is also a method of reducing ringing artifacts by correcting phase jumps observed in phase values of FID signals of water (refer to, for example, Non-patent document 3). In Non-patent document 3, points of extremes of intensities as absolute values of the FID signals of water within a time domain are considered as a phase jump generation region. Further, the ranges to be removed as phase jumps are determined by using a primary derivative for time t of the phase values of the FID signals of water. Specifically, they are determined by fitting regions of the primary derivative around the aforementioned phase jump generation region with a model function, and using full widths at half maximum (FWHM) of the model function obtained by the fitting. Then, correction of phase values of the determined ranges is performed for the phase values.